1. Field of the Invention
The present invention relates generally to a film capacitor element, a film capacitor, and a method of producing the film capacitor element, more particularly, to a film capacitor element including a vapor-deposition metal film layer and a dielectric covering film layer integrally formed by vapor-deposition polymerization or coating on at least one of the opposite major surfaces of a base dielectric film layer, a method of advantageously producing the film capacitor element, and a film capacitor obtained by using the film capacitor element or elements.
2. Discussion of Related Art
Conventionally, film capacitors are used in various kinds of electric devices. There are two types of film capacitor: a laminar film capacitor and a wound type film capacitor. JP-A-10-208972 discloses an example of a laminar film capacitor which is obtained by laminating a plurality of metallized films each including a dielectric film layer formed of a resin film, and a vapor-deposition metal film layer as an electrode film formed on one surface of the dielectric film layer. JP-A-2008-91605 discloses an example of a wound type film capacitor which is obtained by laminating and winding a plurality of the above-described metallized films.
Electric devices are now increasingly required to have a smaller size and higher performance. Along with this requirement, film capacitors are also increasingly required to have a smaller size and higher capacity.
JP-A-2011-61191 discloses an example of a film capacitor which can meet such a requirement and which can be obtained by using a film capacitor element including a base dielectric film layer formed of a resin film and the like, a vapor-deposition metal film layer formed on at least one of the opposite major surfaces of the base dielectric film layer, and a dielectric covering film layer which is formed of a vapor-deposition polymer film and integrally formed on the vapor-deposition metal film layer. A laminar film capacitor is obtained by laminating a plurality of film capacitor elements having the above-described structure and by forming external electrodes on the opposite end faces of the film capacitor. A wound type film capacitor is obtained by winding the film capacitor element or film capacitor elements laminated on each other, and by forming external electrodes on the opposite end faces of the film capacitor.
In both of the laminar film capacitor and the wound type film capacitor, the dielectric covering film layer is formed of a vapor-deposition polymer film, which is integrally formed on the vapor-deposition metal film layer. Therefore, the dielectric covering film layer has an extremely smaller and more uniform thickness than a dielectric covering film layer formed of a resin film. Accordingly, the entire film capacitor element has an extremely smaller and more uniform thickness, and impurities contained in the dielectric covering film layer are sufficiently reduced. Thus, it is possible to advantageously obtain a film capacitor having a smaller size and higher capacity while maintaining a sufficiently high withstand voltage.
A recent advancement of coating techniques makes it possible to form a dielectric coating film having a sufficiently reduced thickness. By integrally forming the dielectric covering film layer formed of the dielectric coating film on the vapor-deposition metal film layer of the film capacitor element, it is possible to obtain a film capacitor having a smaller size and a higher capacity.
In the film capacitor element of the above-indicated publication JP-A-2011-61191, a covering portion is formed in a margin portion formed in an end portion of the base dielectric film layer. The covering portion is integrally formed of the dielectric covering film layer which consists of a dielectric film that covers the end face of the vapor-deposition metal film layer on the side of the margin portion and a corner defined by the above-indicated end face and a portion of the surface of the vapor-deposition metal film layer on which the dielectric covering film layer is formed. As a result, the vapor-deposition metal film layer, which is formed as an internal electrode in the film capacitor element, in contact with one of a pair of external electrodes formed on respective opposite end faces of the wound or laminar film capacitor element, can be stably prevented from contacting with the other external electrode.
However, when the covering portion which consists of a part of the dielectric covering film layer is formed in the margin portion, the following problems are encountered.
In a film capacitor, which is obtained by laminating a plurality of metallized films or winding the obtained laminar body, rather than by using film capacitor elements each including the dielectric covering film layer consisting of a vapor-deposition polymer film or the dielectric coating film, a gap is formed in each margin portion, which is formed between the adjacent metallized films. A portion of the external electrode extends into the gap so as to increase the adhesiveness of the external electrode to the corresponding end face of the film capacitor. However, when a covering portion consisting of the dielectric covering film layer is integrally formed in the margin portion, no gap is formed in the margin portion into which a portion of the external electrode can extend, making it difficult to increase the adhesiveness of the external electrode to the end face of the film capacitor.
In order to overcome this problem, it is considered to form a margin portion having a larger width than the covering portion, thereby forming a gap in the margin portion on one side of the covering portion, so that a portion of the external electrode can extend into the gap. However, in such a case, the following problem is encountered.
In order to form the margin portion having a larger width, it is required that before the vapor-deposition metal film layer is formed on the base dielectric film layer, an oil layer is formed in an end portion of the base dielectric film layer over a large width to prevent the vapor-deposition metal film layer from adhering to the base dielectric film layer. Accordingly, the required amount of oil adhering to the end portion of the base dielectric film layer increases, giving rise to a risk that the oil adheres to the dielectric covering film layer while the film capacitor element is taken up onto or fed from a reel, and adversely affects the dielectric covering film layer. Further, an excessive amount of oil adhering to the dielectric covering film layer increases a possibility that the dielectric covering film layer peels off from the vapor-deposition metal film layer.